Not applicable.
This invention relates generally to receiver circuits and more particularly to radio frequency (RF) receiver circuits.
As is known in the art, a radar system generally includes an antenna, a transmitter and a receiver. In general overview, the transmitter generates an electromagnetic signal which is emitted or radiated through the antenna. The radiated electromagnetic signal propagates in a predetermined region of space and intercepts one or more objects in the path of the electromagnetic radiation. Portions of the electromagnetic radiation reflect off the objects and propagate back towards the radar system where the reflected signals are detected by the receiver. Such reflected signals are sometimes referred to as return or echo signals.
If the radar system employs a directive antenna, a relatively narrow beam of electromagnetic radiation is emitted and the direction from which the return signals propagate and hence the bearing of the object may be estimated. The distance or range to the reflecting object can be estimated by transmitting signal pulses and measuring the time period between the transmission of the transmitted pulse and reception of the return signal pulse.
One particular type of radar system is a monopulse radar system. A monopulse radar system refers to a radar system which obtains a complete measurement of an object""s angular position by transmitting a single signal pulse and receiving the corresponding return or echo pulse. Together with a range measurement performed with the same pulse, the object position in three dimensions is determined completely. Typically, a series or train of echo pulses is employed to make a large number of repeated measurements and produce a refined estimate of the object""s position.
A monopulse receiving system typically includes a monopulse circuit which receives signals from the antenna and forms sum and difference monopulse output signals. The sum and difference signals are formed by combining received antenna signals in a particular manner. The signals can be combined using circuits referred to as hybrid circuits. The hybrid circuits may be provided as so-called magic-T or rat race circuits which receive signals fed thereto and add and/or subtract the signals in a known manner. Such hybrid circuits can be fabricated using either printed circuit or waveguide transmission lines.
To determine the location of an object in a single angular coordinate (e.g. either azimuth or elevation), the monopulse circuit need only include a single hybrid circuit and thus the monopulse circuit is relatively compact. To determine the location of an object in two angular coordinates (e.g. both azimuth and elevation), the monopulse circuit requires multiple hybrid circuits. Thus, conventional monopulse circuits capable of determining the location of an object in two angular coordinates can become relatively large.
The monopulse sum and difference signals can be formed either at the transmitted signal frequency or, after down conversion of a return signal, at a lower frequency. The transmit signal frequency is typically in the microwave or millimeter wave frequency range. When the monopulse sum and difference signals are formed at the transmitted signal frequency, the monopulse is typically coupled directly to the antenna with relatively few, if any, circuits disposed between the antenna output ports and the monopulse input ports. The operations to generate monopulse sum and difference signals typically are performed at microwave or millimeter wave frequencies by the hybrid circuits which are typically fabricated using either printed circuit or waveguide transmission lines.
Obtaining the sum and difference signals at the transmitted signal frequency (i.e., before any down conversion) reduces the amount of additional errors which may be otherwise introduced into the signals used to form the monopulse signals by circuits (e.g. mixer circuits) coupled between the antenna output ports and the monopulse input ports. For example, to form the monopulse signals after down conversion of a return signal to a lower frequency it is necessary to couple a mixer or other frequency translation device between the antenna output ports and the monopulse input ports. Practical frequency translation devices (e.g. mixer circuits) introduce errors into the signals which are combined in the monopulse circuit to provide the monopulse output signals.
Typically, a single sum channel and a pair of difference channels are formed by the monopulse circuit to allow resolution of two angular coordinates. In systems which utilize a conventional waveguide multimode horn feed, a waveguide monopulse network can process a radar return signal to generate monopulse sum and difference signals which propagate in appropriate monopulse sum and difference channels. The radio frequency (RF) signals propagating through the monopulse channels are converted to intermediate frequency (IF) signals using waveguide mixers. The IF signals are fed to an IF receiver for additional processing.
One problem with this RF waveguide approach to implementing the monopulse network is that the monopulse circuit is relatively large and must be fabricated using relatively expensive and time consuming precision machining or electroforming techniques. This is particularly true in those system which operate in the millimeterwave frequency range. To overcome this drawback, systems operating at millimeter wavelength,frequencies can downconvert received signals to an intermediate frequency prior to monopulse processing. With this approach, monopulse processing may be performed at the intermediate frequency in lieu of monopulse processing performed at the higher fundamental or transmit frequency. While the circuit fabrication tolerances are generally less severe at lower frequencies, there is a concomitant increase in the size of waveguide circuit components. Thus, the use of waveguide transmission lines to process and convert the monopulse information (especially at millimeter wave frequencies) is not a practical low cost solution suitable for high volume production.
To further complicate matters, projectiles such as missiles and submunitions having a relatively small diameter require relatively high resolution monopulse receivers to enable accurate tracking of a target. Conventional monopulse receiving systems operating in the 1 gigahertz (GHz) to 20 GHz frequency range do not provide the angular resolution needed to accurately track targets. Furthermore, the size of RF circuit components which operate in the 1 GHz to 20 GHz range are physically too large and cumbersome to be packaged in many small projectiles. Therefore, operation at millimeter wave frequencies above 30 GHz is required.
Missile seeker systems having a relatively large diameter typically operate at microwave frequencies and form monopulse output receive signals with comparator networks provided from hybrid circuit components implemented using stripline, coaxial or waveguide transmission media. The monopulse output signals are typically fed to amplifiers having a relatively high gain and a relatively low noise figure. The amplified signals are subsequently downconverted to an appropriate intermediate frequency (IF) by a radio frequency (RF) microwave mixer module. For those applications in which the monopulse receiver must be disposed in a projectile having a relatively small diameter, however, the signal transmission losses and overall size of conventional receiver systems adversely impact seeker performance. Operation at higher frequencies such as millimeter wave (MMW) frequencies is a necessity to achieve the requisite resolution but there are limitations in the availability of receiver devices which operate at such frequency bands. For example, it is relatively difficult and expensive to provide RF devices having the performance characteristics (e.g., noise figure, power handling, power limiting, etc.) required for efficient active seeker operation in the MMW frequency range.
The complexity of radar systems operating in the millimeter wave frequency band will be appreciated when it is recognized that at an operating frequency of 94 GHz, for example, dimensions of a conventional rectangular waveguide are in the order of 0.050 to 0.100 inches, with tolerances of better than 0.001 inches required in many critical assemblies. Although it may be possible to fabricate such millimeter-wave waveguide structures at somewhat reduced cost using modem fabrication techniques, the expense associated with tuning and testing such critically toleranced hardware is often cost prohibitive.
Furthermore, the problems of packaging and tuning a millimeter-wave seeker in a conventional submunition will be appreciated when it is recognized that a monopulse seeker with a monopulse tracking capability utilizing waveguide components may well require in excess of twenty different waveguide components to control the routing and duplexing of the various signals coming from the transmitter and returning to the receivers. If a monopulse capability were required, then all of the foregoing waveguide components would be required to track from channel to channel in both amplitude and phase.
At an operating frequency of 94 GHz, each one thousandth of an inch in the length of a waveguide transmission line is equivalent to about 2xc2x0 of phase. It should, therefore, be appreciated that it is relatively difficult to obtain inexpensively the requisite phase and amplitude tracking between various receiver channels.
Another problem inherent in millimeter-wave radar seekers utilizing waveguide devices is that of providing sufficient isolation between a transmitter and receiver. This problem is exacerbated by the fact that waveguide switches and circulators which can withstand relatively high power transmit signals and provide a high degree of isolation are not generally available in a compatible size at relatively high operating frequencies.
It would, therefore, be desirable to provide a relatively compact monopulse receiver having a relatively low noise figure which operates in the millimeter wave frequency range and which can operate in a system which includes a transmitter which transmits signals having relatively high power levels.
In accordance with the present invention, a radio frequency (RF) system includes an antenna having a plurality of antenna ports and a plurality of protection circuits each of the protection circuits having a first port coupled directly to a respective one of the plurality of antenna ports and a second port. In response to a first control signal, each protection circuit allows signals to propagate from a respective one of the antenna ports to the second port of the respective protection circuit along a signal path having a relatively low insertion loss characteristic. Each protection circuit is also responsive to a second control signal in a first direction between the first protection circuit port and the second protection circuit port and responsive to a second control signal which isolates the first protection circuit port from the second protection circuit port. With this particular arrangement, a compact RF system is provided. By coupling the antenna ports directly to the ports of the protection circuit, the RF system can operate in a receive mode and be protected from transmit signal having high signal levels generated by a transmitter circuit during a transmit operating mode. To operate in a receive mode, the protection circuit is biased to provide a signal path having a relatively low insertion loss characteristic to signals propagating from the antenna ports through the protection circuit ports. During the transmit mode, the protection circuit is biased to provide a signal path having high insertion loss characteristic from signals propagating from the antenna ports. In one particular embodiment, the RF system further includes a plurality of mixers, each having a first port coupled to a respective one of the plurality of protection circuit ports and a second port for receiving a mixer bias signal and a third port for providing a frequency shifted signal. By coupling the mixers to the protection circuit ports, a compact receiver assembly is provided. An amplifier can be coupled to the third port of each mixer to thus provide a system having a relatively high gain characteristic while also providing a system which provides a relatively low noise figure. Also a monopulse can be coupled to the output ports of the amplifiers to provide an RF monopulse receiving system. In a preferred embodiment, the mixers, amplifiers and monopulse circuit are provided as monolithic microwave integrated circuits (MMICs) and thus the RF system utilizes a relatively small physical area. Furthermore the antenna can be provided as a corrugated horn having a moding structure disposed in a base portion thereof to couple signals between the antenna input and the antenna ports in the base structure of the corrugated horn which are coupled to the first port of each of the protection circuits. The receiving system may also include a calibration signal injection circuit coupled to the protection circuit to inject a calibration signal into the receiving system. In one particular embodiment, the protection circuit is a latching ferrite isolator matrix which includes a plurality of isolators each having first, second and third ports with the calibration signal injection circuit coupled to the third port of each of the plurality of isolators.
In accordance with a further aspect of the present invention, a radio frequency (RF) monopulse receiver includes a plurality of mixers, each having an RF signal port, a local oscillator (LO) signal port and an intermediate frequency (IF) signal port and each of the mixers comprising one or more mixer diode anti-parallel pairs and means for coupling RF energy to the RF signal port of each of the plurality of mixers. The RF monopulse receiver further includes a plurality of IF amplifiers, each of the IF amplifiers having an amplifier input port coupled to the IF signal port of a respective one of the plurality of mixers and an amplifier output port coupled to a respective one of a plurality of input ports of a monopulse comparator network. In response to appropriate input signals fed thereto, the monopulse comparator network provides monopulse output signals at output ports thereof. With this particular technique, a compact millimeter wave monopulse receiver having a relatively low noise figure is provided. By arranging a latching ferrite isolator matrix protection circuit between an antenna and the mixer ports, the RF monopulse receiver is protected from high power transmit signals. Furthermore the latching ferrite isolator matrix allows use of a receiver circuit architecture which allows the compact millimeter wave monopulse receiver circuit to operate in an RF radar system having a relatively high transmit power. In one embodiment, the RF monopulse receiver is suitable for use in an active missile seeker system for example. The RF receiver can be used directly in a small submunition or alternatively, can function as the monopulse receiver for a higher resolution quasi-optically fed, antenna having an aperture much larger than the diameter of the horn antenna. In one embodiment, a cryogenic cooling system is coupled to the receiver to provide a receiver noise figure which is lower than the noise figure achieved when the receiver operates at ambient temperatures.
In accordance with a still further aspect of the present invention, an RF monopulse receiver includes a circuit assembly having a plurality of RF input ports and a plurality of IF output ports. The circuit assembly includes (a) a housing, (b) a plurality of subharmonically pumped mixer circuits disposed in the housing, each of the mixer circuits having an RF signal port, an LO signal port and an IF signal port and each of the mixer circuits including: (1) a plurality of mixer diode substrates disposed in the housing, each of the mixer diode substrates having a diode mounting region and a transmission coupling region which projects into an RF feed region which is formed by providing an opening in a housing cover disposed over the diode mounting region of the plurality of substrates; (2) an antiparallel diode pair disposed on the diode mounting region of each of the plurality of mixer diode substrates; (3) an LO distribution circuit coupled between the LO signal port of each subharmonically pumped mixer circuit and an LO signal source; (4) an IF distribution circuit coupled between the IF signal port of each subharmonically pumped mixer circuit and an IF output port of the RF monopulse receiver; (c) an RF feed circuit, coupled to the housing, (d) means for coupling RF energy to the RF signal port of each of the plurality of subharmonically pumped mixer circuits; (e) a monopulse substrate; and (f) a monolithic microwave integrated circuit (MMIC) monopulse comparator network disposed on the monopulse substrate, the MMIC monopulse comparator network having a plurality of monopulse circuit input ports, each of the plurality of monopulse circuit input ports coupled to a respective one of the IF ports of the plurality of subharmonically pumped mixer circuits and having a plurality of monopulse circuit output ports coupled to the IF output ports of the monopulse substrate. With this particular arrangement, an RF monopulse receiver suitable for use in the W-band frequency range is provided. The system can further include a corrugated horn antenna having a moding structure in a base portion thereof to provide four separate antenna ports. The antenna base ports are coupled to the RF ports of the mixer circuits providing signals between the antenna ports in the base structure of the corrugated horn and the RF port of the mixer circuits. A protection circuit can be included in a waveguide signal path disposed between the antenna ports and the RF input port of the mixer circuits. In one particular embodiment, the protection circuit is provided as a latching ferrite isolator matrix which includes a plurality of isolators each having first, second and third ports. The RF monopulse receiver can also include a calibration signal injection circuit coupled to the protection circuit to inject a calibration signal into the receiver. When the protection circuit is provided as the latching ferrite isolator matrix, a calibration signal injection circuit can be coupled to the third port of each of the plurality of isolators. In response to a first control signal, the latching ferrite isolators allow signals to propagate in a first direction between a first pair of isolator ports and in response to a second control signal, the latching ferrite isolators allow signals to propagate in a second direction between a second pair of isolator ports. With such an arrangement, the RF monopulse receiver can operate without being damaged in those RF systems which include a transmitter. Furthermore, the housing may be provided as a single housing having both RF and IF circuit components disposed therein including the MMIC monopulse comparator network. Alternatively, the housing can be provided from an RF housing and an IF housing which are physically and electrically coupled together. The RF signal components including the RF mixer and an amplifier are disposed in the RF housing and the MMIC monopulse comparator network and an amplitude adjustment circuit and phase shifter are disposed in the IF housing with a plurality of RF interconnect signal paths providing RF signal paths between the RF housing and the IF housing. The use of MMIC LNA""s and a novel MMIC monopulse along with the development of a low conversion loss, low noise figure, W-Band mixer leads to a relatively small and efficient W-Band monopulse receiver which is compatible with small diameter missiles and submunitions and provides enhanced sensitivity to thus allow the seeker to accurately track targets. This enhanced sensitivity is achieved with the inclusion of cooling hardware which is used to locally cool the mixer diodes.